The present invention relates to a method and a device for reducing crosstalk interference in a transmission system which makes use of frequency translated signals, particularly a discrete multi tone (DMT) modulated transmission system or an OFDM (Orthogonal Frequency Division Multiplex) transmission system, in. which the modulation may be effected using a fast inverse Fourier transform (IFFT). The invention relates further to the transmission system itself, and to a transceiver of said transmission system.
Digital data duplex transmission systems are currently being developed for high-speed communication. Standards for high-speed data communication over twisted-pair phone lines that have been developed include Asymmetric Digital Subscriber Lines (ADSL) and Very High Speed Digital Subscriber Lines (VDSL).
A standardized ADSL system (ANSI T1.413-1995, ATIS Committee T1E1.4), providing transmission at rates up to 8 Mbit/s over twisted-pair phone lines, defines the use of a discrete multi tone (DMT) system that uses 256 carriers or sub-channels, each 4.3125 kHz wide, in the downstream direction. In this context the downstream direction is defined as transmission from a central office (typically owned by a telephone company) to a remote location such as an end-user (i.e. residence or business user). The standard defines also the use of an oppositely directed (i.e. in the upstream direction) signal at a rate of 16 to 800 kbit/s, which is considerably lower than in the downstream direction.
A corresponding VDSL standard is intended to provide transmission up to 25.96, and preferably up to 51.92 Mbit/s, in the downstream direction and requires generally shorter phone lines than what is permitted using ADSL. Another system that is similar to VDSL is referred to as Fiber To The Curb (FTTC).
Several modulation schemes have been proposed for use in the standards mentioned above, of which most use frequency division multiplexing of the upstream and downstream directions. Other modulation systems proposed for the VDSL and FTTC systems, including multi-carrier transmission schemes such as DMT and single carrier transmission systems such as Quadrature Amplitude Modulation (QAM), use non-overlapping periodic synchronized upstream and downstream communication periods separated by a silent period. Such a system is referred to as a xe2x80x9cping pongxe2x80x9d based data transmission system.
A common feature of all the above mentioned systems is that twisted-pair phone lines are used at least a part of the transmission medium that connects a central office (e.g. telephone company) to users (e.g. residence or business). It is difficult to avoid twisted-pair wiring from all parts of the interconnecting transmission medium. Even though fiber optics may be available from a central office to a curb near a user, twisted-pair phone lines are used to bring signals from the curb into the users residence or business.
The twisted-pair phone lines are grouped in a binder. While the twisted-pair phone lines are within the binder, the binder provides reasonably good protection against external electromagnetic interference. However, within the binder, the twisted-pair phone lines, being located close together, induce electromagnetic interference on each other. This type of electromagnetic interference is known as crosstalk interference. As the frequency of transmission increases, the crosstalk becomes substantial. As a result, the data signals being transmitted over the twisted-pair phone lines at high speeds can be significantly degraded by the crosstalk caused by other twisted-pair phone lines in the binder. As the speed of data transmission increases, the problem gets worse.
Conventional crosstalk cancelers have been used to reduce crosstalk. The difficulty with such conventional crosstalk cancelers is that they are very complex and consume large amounts of resources. For instance, the approach described in M. L. Honig et al., xe2x80x9cSuppression of Near- and Far-end Crosstalk by Linear Pre- and Post-filtering, xe2x80x9cIEE Journal on Selected Areas in Communication, Vol. 10, No. 3, pp. 614-629, April 1992, requires so much processing to implement the filtering that its benefits are overshadowed by the processing burdens.
PCT application WO 98/10528 (inventor J. F. Cioffi) proposes a system for removing crosstalk by adaptively estimating the crosstalk as induced by other of the interfering lines and canceling the crosstalk by using the estimated crosstalk. The adaptive scheme avoids processing when it is not justified in view of its processing costs. The document does not, however, address problems associated with the computational complexity of the reduction as such.
Hence, the problem of using twisted-pair phone lines at high frequency data transmission rates, such as those available using ADSL and VDSL, is that crosstalk, particularly NEXT from other lines in a binder, becomes a substantial impediment to proper reception of the transmitted data signals. Conventional NEXT cancelers are complex and do need considerable processing power for implementing the reduction.
Also in orthogonal frequency division multiplexed (OFDM) transmission systems, crosstalk may occur which systems correspondingly also needs large amounts of processing power to handle the crosstalk.
It is consequently an object of the present invention to provide a method for reducing crosstalk interference in a transmission system which makes use of frequency translated signals, particularly a discrete multi tone (DMT) modulated transmission system or an OFDM (Orthogonal Frequency Division Multiplex) transmission system, that uses less computational power as compared with known practice.
It is a further object of the invention to provide the estimated crosstalk interference at a fast rate.
It is yet a further object of the invention to provide the reducing of crosstalk interference as being implemented at least partly in specific hardware.
These objects among others are, according to one aspect of the invention, fulfilled by a method for reducing crosstalk interference induced on a signal on a first line by a signal on a second line, comprising following steps. A complex coupling factor for the crosstalk interference, which when multiplied by the signal on the second line estimates the induced crosstalk interference, is estimated, the complex coupling factor is multiplied by the signal on the second line through an approximation method operating on the signal on the second line including pre-rotation, scaling and multiplication by a complex number according to the coupling factor, whereby the complex number is chosen from a bank of predetermined complex numbers so as to obtain the best approximation possible. Finally, the product, i.e. the estimated induced crosstalk interference, obtained is subtracted from the signal on the first line.
The number of complex number provided in the bank may be calculated from a maximum acceptable error in the computation.
According to a second aspect of the present invention, there is provided a method for reducing crosstalk interference induced on a signal SN on a first line N by signals D1, . . . , DNxe2x88x921, each on a respective line 1, . . . , Nxe2x88x921.
The method comprises following steps. A coupling factor xcex1xe2x80x21xe2x80x94N, . . . , xcex1xe2x80x2Nxe2x88x921xe2x80x94N is associated with the respective line 1, . . . , Nxe2x88x921, said coupling factor xcex1xe2x80x2jxe2x80x94N, 1xe2x89xa6jxe2x89xa6Nxe2x88x921, being a complex number and when multiplied by the signal Dj, 1xe2x89xa6jxe2x89xa6Nxe2x88x921, on its associated line j, 1xe2x89xa6jxe2x89xa6Nxe2x88x921, estimating the crosstalk interference Ijxe2x80x94N induced on the signal SN on the first line N by the signal Dj on its associated line j. The crosstalk interference IN on the signal SN on the first line N is reduced by subtracting an estimated crosstalk interference Ixe2x80x2N from the signal SN on the first line N, said estimated crosstalk interference Ixe2x80x2N being computed from said coupling factors xcex1xe2x80x21xe2x80x94N, . . . , xcex1xe2x80x2Nxe2x88x921xe2x80x94N, and said signals D1, . . . , DNxe2x88x921 on the respective line 1, . . . , Nxe2x88x921 in the manner described below.
Each of the signals D1, . . . , DNxe2x88x921 on the respective line 1, . . . , Nxe2x88x921 is pre-rotated and scaled according to the coupling factor xcex1xe2x80x21xe2x80x94N, . . . , xcex1xe2x80x2Nxe2x88x921xe2x80x94N associated with the respective line, whereafter all of the pre-rotated and scaled signals D*1, . . . , D*Nxe2x88x921, obtained are summed. The sum xcexa3D* obtained is multiplied by a single complex number xcex2N and the product obtained is used as the estimated crosstalk interference Ixe2x80x2N in said above subtraction.
According to a third aspect of the present invention there is provided a method for reducing crosstalk interferences induced by a signal DN on a first line N on signals S1, . . . , SNxe2x88x921 on a respective line 1, . . . , Nxe2x88x921, comprising following steps.
Coupling factors xcex1xe2x80x2Nxe2x80x941, . . . , xcex1xe2x80x2Nxe2x80x94Nxe2x88x921 are associated with the first line N; each of said coupling factors is a complex number and when multiplied by the signal DN on the first line N it estimates the respective crosstalk interference INxe2x80x941, . . . , INxe2x80x94Nxe2x88x921 induced on each of the signals S1, . . . , SNxe2x88x921 on the respective line 1, . . . , Nxe2x88x921 by the signal DN on the first line N. The respective crosstalk interference INxe2x80x941, . . . , INxe2x80x94Nxe2x88x921 on each of the signals S1, . . . , SNxe2x88x921 on the respective line 1, . . . , Nxe2x88x921 is reduced by subtracting a respective estimated crosstalk interference Ixe2x80x2Nxe2x80x941, . . . , Ixe2x80x2Nxe2x80x94Nxe2x88x921 from each of the signals S1, . . . , SNxe2x88x921 on the respective line 1, . . . , Nxe2x88x921, said respective estimated crosstalk interference Ixe2x80x2Nxe2x80x941, . . . , Ixe2x80x2Nxe2x80x94Nxe2x88x921 being computed from the respective coupling factor xcex1xe2x80x2Nxe2x80x941, . . . , xcex1xe2x80x2Nxe2x80x94Nxe2x88x921, and the signal DN on the first line N in the manner described below.
The signal DN on the first line N is multiplied by a single complex number xcex2N yielding a product D**N, whereafter the product D**N is replicated for obtaining Nxe2x88x921 equal products D**Nxe2x80x941, . . . , D**Nxe2x80x94Nxe2x88x921. The respective product D**Nxe2x80x941, . . . , D**Nxe2x80x94N xe2x88x921 is pre-rotated and scaled according to the respective coupling factor xcex1xe2x80x2Nxe2x80x941, . . . , xcex1xe2x80x2Nxe2x80x94Nxe2x88x921, and, finally, the respective pre-rotated and scaled products obtained are used as the respective estimated crosstalk interference Ixe2x80x2Nxe2x80x941, . . . , Ixe2x80x2Nxe2x80x94Nxe2x88x921 in the respective subtraction.
Preferably, the pre-rotating is performed by mirroring in the real and/or the imaginary and/or the 45xc2x0 axis.
The complex multiplications may be performed by using a vector rotation method, particularly the CORDIC (COordinate Rotation DIgital Computer) vector rotation method.
The scaling is preferably performed by multiplying by real numbers chosen from a bank of predetermined real numbers so as to obtain the best approximation possible.
Furthermore, the invention comprises a device and a transmitter adapted for performing the method according to any of the three first aspects of the invention, and a transmission system comprising a device or a transmitter of above said kind.
An advantage of the present invention is that the computation of the estimated crosstalk interference is simple and rapid, thus permitting adaptive reduction of crosstalk interference in real time to accommodate, for example, changing transmitting conditions.
Another advantage of the invention is that the simplified procedures allows, at least partly, implementation using dedicated hardware for obtaining improved speed.